Inter-exchange soft handoff in a cellular telecommunications system

ABSTRACT

In a cellular telecommunications system having mobile exchanges, base stations and mobile stations, the mobile exchanges are capable of both intra-exchange and inter-exchange soft handoffs. Thereby, a seamless soft handoff with signal diversity combining is obtained throughout the service area of the system, which preferably is a CDMA system.

FIELD OF THE INVENTION

The present invention relates to cellular telecommunications systems.More specifically, the present invention relates to a novel and improvedsystem for soft handoff between a mobile station and base stations whichare connected to different mobile switching centers within a cellulartelecommunications system.

BACKGROUND OF THE INVENTION

The use of code division multiple access (CDMA) modulation is but one ofseveral techniques enabling digital communications among a number ofmobile users utilizing a common part of the radio spectrum, as is thecase for cellular telecommunications systems. Other well-known radioaccess techniques are time division multiple access (TDMA) and frequencydivision access (FDMA). The concept of soft handoff to which the presentinvention is closely related, is indeed applicable to all three of thementioned multiple access techniques and will result, if applied insteadof the conventional hard handoff schemes, in increased system capacityand fewer dropped calls. However, soft handoff is mandatory for CDMA, asthe use of conventional hard handoff would result in a very poor systemperformance. The background of the present invention will be presentedfor a CDMA cellular telecommunications system. However, it should beunderstood that the present invention is not limited to CDMA. Anexemplary application of CDMA to cellular telecommunications systems hasbeen substantially described in "On the System Design Aspects of CodeDivision Multiple Access (CDMA) Applied to Digital Cellular and PersonalCommunications Networks", Allen Salmasi and Klein S. Gilhousen,presented at the 41st IEEE Vehicular Technology Conference on May 19-22,1991 in St. Louis, Mo.

In the above mentioned publication, a direct-sequence CDMA (DS-CDMA, or,for short, CDMA in the following) technique is described in which anumber of user mobile stations (MSs) communicate via CDMA radio spreadspectrum signals with base stations (BSs, also referred to as cellsites) in the uplink (mobile station to base station) and downlink (basestation to mobile station) direction. The base stations convert theseCDMA radio signals respectively originating from, and terminating at,the user's MS into a form appropriate for use in conjunction withterrestrial telecommunications transmission equipment such as thecommonly deployed Pulse Code Modulation (PCM) circuit facilities. Thebase stations further relay these user signals in the uplink anddownlink directions to a mobile switching center (MSC, also referred toas a mobile exchange or mobile telephone switching office (MTSO)) forfurther processing.

The above-mentioned user communication signals comprise digitized voicesignals and control information (also referred to as signalling). TheMSC performs multiplexing and conversion operations on theabove-mentioned tributaries, and relays the voice signal to anotheruser, e.g. within the Public Switched Telephone Network (PSTN). The MSCalso interprets, reacts upon, and generates signalling information, thuscontrolling the overall communication links between the users. Thesecommunications link control functions comprise the management of generalcall related events such as call setup and tear down, as well as CDMAradio link-related events such as deterioration of CDMA radio linkquality, and subsequent handoff initiation.

If CDMA is deployed within the typical medium, to large-sized cells ofland mobile telecommunications systems, then the average time delayspread of the multipath radio propagation environment is usually longerthan the duration of each chip of the DS-CDMA signal. This forces CDMAto operate in an asynchronous mode, with the consequence that theorthogonality of the spread spectrum multiple access user signals cannotbe achieved by means of orthogonal spreading codes alone. Therefore, thecommunications suffer from system self-induced interference, not onlyamong signals originating from different cells, but in addition to that,also, considerably, within a single cell (referred to as CDMA intra-cellinterference). For such CDMA cellular systems, it is therefore animportant overall system design objective to minimize any excessive CDMAinterference among the communicating users and it is a complementarydesign objective, to capture and utilize as much energy from a desiredCDMA user signal as possible. This system design requirement, although ageneric requirement applicable to any multiple access method withincellular telecommunications systems, is less stringent for FDMA and TDMAbased systems in which intra-cell interference is avoided by theintrinsic properties of the respective multiple access method andinter-cell interference is limited by means of pre-planned cellularfrequency re-use schemes. Thus, CDMA unlike FDMA or TDMA operates in astrictly interference-limited manner.

Nevertheless, soft handoff will also improve the TDMA system capacity.However, the gain will be less than for a CDMA system. In the following,the invention is for the case of a CDMA cellular telecommunicationssystem.

Several methods for implementing the above-mentioned CDMA system designobjective can be readily identified for the above-referenced exemplaryembodiment of a CDMA cellular telecommunications system. For example,the described closed loop MS transmit power control method has theobjective to continuously equalize the received qualities of all uplinkCDMA signals within a single BS against the background of rapidlychanging radio propagation channels undergoing fast and slow fadingprocesses. For this purpose, the BS measures periodically the receivedE_(b) /N_(o) value, indicative of the signal quality, from each MS CDMAuplink communication and subsequently transmits an appropriate powercontrol command on the downlink communication channel to the MS which,in turn sets the CDMA transmitter power accordingly- Ideally, all MSCDMA uplink signals are received at the BS with the same quality, and,in addition to that, minimum strength necessary in order to maintain thecommunication link subject to a predetermined quality threshold.

Another embodiment of the previously mentioned system design objectiveis the method of mobile assisted soft handoff in conjunction with signaldiversity combining during an active CDMA communication which will besummarized in the following and to which the present invention isclosely related.

Mobile assisted soft handoff in conjunction with signal diversitycombining comprises the method of relaying user communication signals onthe transmission segment between MS and MSC concurrently via a first anda second BS in the uplink and downlink direction and performing signaldiversity reception at the MS and MSC in order to enhance the usersignal quality. This method is invoked by the MSC when an MScommunicating initially with a BS has moved into the overlappingcoverage areas of this first BS and a second BS and has reported theavailability of a sufficiently strong signal from this second BS to theMSC. At no time instant during soft handoff in conjunction with signaldiversity combining does the MS interrupt its communications with theMSC. The MSC typically deploys postdetection/decoding, selectivecombining of the digitally encoded speech frames.

In order to enable the reporting assistance of the MS during softhandoff initiation, all BSs may transmit a CDMA downlink referencesignal, referred to a pilot signal. MSs when roaming throughout theservice area of the CDMA cellular telecommunications system periodicallydemodulate the pilot signals of the various neighboring BSs during anongoing communication with a first BS and derive a corresponding pilotsignal quality indication. Again, the measured pilot E_(b) /N_(o) maycome as a signal strength/quality indication. This indication determinesa ranked list of candidate BSs for handoff and is transmitted in theform of signalling information to the MSC. It should be understood that,also the first BS may perform continuously CDMA uplink signal qualitymeasurements, and, based upon these observations, may give a softhandoff request indication to the MSC.

Usually, soft handoff in conjunction with signal diversity combining isinitiated by the MSC if the MS reports that the pilot signal quality ofa second BS in addition to that of the first BS is sufficiently goodaccording to the predetermined thresholds made available to the MS . TheMSC, as well as the second BS, can obtain the required resources for thesoft handoff transition. Subsequently, the MS will be instructed by theMSC via the first BS by means of signalling to initiate a soft handoffand to commence signal diversity combining on the downlink.

Moreover the MSC initiates the additional relay of user signals via thesecond BS and commences diversity combining of the user signal in theuplink direction. Both participating BSs invoke autonomously thepreviously mentioned closed loop power control method. The MS sets itsCDMA transmit power to the minimum of the two commanded power levels inorder to reduce excessive CDMA interference with the other communicationlinks.

Finally, when the MS is firmly established within the area of the secondBS and the pilot signal received from the first BS has weakenedsufficiently according to the predetermined thresholds made available tothe MS it the MS will report this condition to the MSC which in turndecides to terminate the soft handoff with signal diversity combiningand will use subsequently only the second BS for maintaining the CDMAcommunications.

This process of soft handoff with signal diversity combining may berepeated as the MS moves within the service area of the CDMA cellulartelecommunications system and as the measured CDMA signal qualityindications suggest.

Some of the contemporary TDMA based cellular telecommunications systemsalso utilize the assistance of the MS in form of corresponding MSdownlink signal quality measurements as a trigger for requesting ahandoff from a first BS to a second BS, much in the same way assummarized above. However, these systems usually use a scheme referredto as hard handoff, in which the MS, in response to instructions fromthe MSC disrupts the communication with the first BS, tunes into theindicated TDMA radio channel of the second BS, and then resumes theuplink and downlink communications. At no time instant does the MScommunicate with more than one BS simultaneously and hence, nocorresponding signal diversity combining takes place in the MS nor inthe MSC as is the case for the soft handoff method described above. Thishard handoff scheme is applicable to CDMA in a similar manner as well,but should be avoided whenever possible, for reasons of CDMA systemcapacity as explained in the following. As mentioned above, though lessuseful, the inter-exchange soft handoff is applicable to TDMA and FDMAas well.

Within the context of soft and hard handoff the previously mentionedpredetermined thresholds used in conjunction with the MS downlink signalquality measurements for the determination of handoff candidate BSs arealso referred to as handoff margins. The use of these handoff margins inconjunction with time a hyphen averaging processes is necessary in orderto avoid frequent handoffs (also referred to as handoff Ping-Pongeffect) when the MS moves within the usually fuzzy border between theradio signal coverage areas of two neighboring BSs. Such frequenthanddoffs would overload the processing capacity of the MSCs. For thepurpose of controlling the CDMA soft handoff with signal diversitycombining, the handoff margin can be chosen, as small as 1-3 dB, incontrast to the hard handoff case where usually 6-10 dB are required inorder to avoid the deleterious handoff Ping-Pong effect.

Referring to the previously mentioned interference limited operation ofCDMA, small handoff margins are indeed an essential requirement for anefficient operation of CDMA. The use of CDMA hard handoff in conjunctionwith the necessary large hard handoff margins would substantiallydecrease the CDMA system capacity. In a CDMA cellular telecommunicationssystem, hard handoff can be tolerated only in exceptional situations butnot as a normal mode of system operation. Soft handoff with signaldiversity combining therefore is to be provided on a seamless basisthroughout the CDMA system service area.

Prior art CDMA cellular telecommunications systems provide soft handoffwith signal diversity combining only between BSs which are connected toone and the same MSC (referred to as intra-MSC soft handoff with signaldiversity combining). If the MS is to be handed off between BSsconnected to different MSCs, prior art CDMA cellular telecommunicationssystems use CDMA hard handoff instead with the above, mentioneddisadvantages from the capacity point of view.

SUMMARY OF THE PRESENT INVENTION

It is an object of the present invention to provide a system and amethod for accomplishing soft handoff with signal diversity combiningamong base stations connected to different mobile exchanges of acellular telecommunications system, in the following referred to asinter-exchange soft handoff with signal diversity combining. The presentinvention thus relates to a system and a method to provide seamless softhandoff throughout the service area of a whole cellulartelecommunication system.

One aspect of the invention is, in a cellular telecommunications systemin which a user mobile station relays user information radio signals viaat least one of a plurality of base stations and in which the basestations further relay the user information signals via at least one ofa plurality of mobile exchanges to and from another system user, amethod for inter-exchange soft handoff with diversity combining,comprising the steps of:

maintaining the relay of user information signals between a mobilestation and a first mobile exchange via a first base station connectedto the first mobile exchange, the first mobile exchange further relayingthe user information signals to and from another system user, the firstmobile exchange controlling the user communications,

establishing a further second connection for further second relay of theuser information signals between the mobile station and the first mobileexchange via a further second base station connected to a further secondmobile exchange, the second mobile exchange further relaying said thecommunications signals to the first mobile exchange,

performing diversity combining at the first mobile exchange of said userinformation signals as relayed from the mobile station to the firstmobile exchange via the first connection and the further secondconnection, the first mobile exchange offering the combined userinformation signal to the other system user,

performing further diversity combining at the mobile station of said theinformation signals as relayed from the other system user to the firstmobile exchange and relayed further via the first connection and saidfurther second connection to the mobile station.

The invention enables accomplishment an inter-exchange soft handoff, andthereby allows accomplishment a seamless soft handoff with signaldiversity combining throughout the service area of the system.

Other well-known radio access techniques are time division multipleaccess (TDMA) and frequency division access (FDMA) . The concept of softhandoff to which the present invention is closely related, is indeedapplicable to all three of the above mentioned multiple accesstechniques and will result, if applied instead of the conventional hardhandoff schemes, in increased system capacity and fewer dropped calls.However, soft handoff is mandatory for CDMA, as the use of conventionalhard handoff would result in a very poor system performance. For thesereasons, the preferred embodiment of the present invention is in a CDMAcellular telecommunications system. However, it should be understoodthat the present invention is not limited to CDMA.

One aspect of the invention is a cellular telecommunications systemcomprising a plurality of mobile exchanges, a plurality of base stationsand a plurality of mobile stations roaming throughout the system area,in which a user mobile station relays user information radio signals viaat least one of a plurality of base stations and in which the basestations further relay the user information signals via at least one ofa plurality of mobile exchanges to and from another system user, thesystem of inter-exchange soft handoff with diversity combining, thesystem comprising:

means at each mobile station of the plurality of mobile stations and ateach base station of the plurality of base stations and at each mobileexchange of the plurality of mobile exchanges, to maintain the relay ofuser information signals between a mobile station and a first mobileexchange via a first base station connected to the first mobileexchange, further means at the first mobile exchange for furtherrelaying the user information signals to and from another system user,further means at the first mobile exchange for controlling the usercommunications,

means at each mobile station of the plurality of mobile stations and ateach base station of the plurality of base stations and at each mobileexchange of the plurality of mobile exchanges for establishing a furthersecond connection for further second relay of the user informationsignals between the mobile station and the first mobile exchange via afurther second base station connected to a further second mobileexchange, the second mobile exchange comprising further means forfurther relaying the user communications signals to said first mobileexchange,

means at the first mobile exchange for performing combining of the userinformation signals as relayed from the mobile station to the firstmobile exchange via the first connection and the further secondconnection, the first mobile exchange comprising further means foroffering the combined user information signal to the other system user,

means at the mobile station for performing further diversity combiningof the user information signals as relayed from the other system user tothe first mobile exchange and relayed further via said first connectionand the further second connection to the mobile station.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present invention will become moreapparent from the detailed description set forth below, when taken inconjunction with the attached drawings in which:

FIG. 1 is a schematic overview of an exemplary CDMA cellulartelecommunications system in accordance with the present invention;

FIG. 2 is a block diagram showing a preferred embodiment of a mobileswitching center for use within a CDMA cellular telecommunicationssystem;

FIG. 3 is a block diagram showing a preferred embodiment of a basestation for use within a CDMA cellular telecommunications system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an exemplary embodiment of a CDMA cellulartelecommunications system to which the present invention relates. Thesystem illustrated in FIG. 1 improves upon the prior art CDMA softhandoff and macrodiversity signal combining techniques, as those aresubstantially described in "On the System Design Aspects of CodeDivision Multiple Access (CDMA.) Applied to Digital Cellular andPersonal Communications Networks", Allen Salmasi and Klein S. Gilhousen,presented at the 41st IEEE Vehicular Technology Conference on May 19-22,1991 in St. Louis, Mo.

FIG. 2 shows an exemplary embodiment of an MSC used in a CDMA cellulartelecommunications system to which the present invention relates.

Digital links (120,122,124,126) connect the mobile exchange MSC with thePublic Switched Telephone Network PSTN, other mobile exchanges MSC, andbase stations BS, respectively. These digital links carry the userinformation, such as voice and, additionally, signalling information.The preferred embodiment of the present invention assumes that tinesignalling information on is multiplexed together with the userinformation onto one and the same physical transmission facility. T1transmission facilities together with Signalling System No. 7 may serveas an exemplary embodiment of such a digital link arrangement.

The user information stream is switched among the herein mentionedentities by means of a digital switch 112. The corresponding signallinginformation is transmitted, received and relayed by a packet switch 114.Packet switch 114 is also connected to an MSC control processor 110which acts as a signalling information source and sink, respectively.The MSC control processor 110 interprets and reacts upon signallingmessages addressed to it and may also solicit signalling messages toother entities, whenever appropriate the MSC control processor 110 alsocontrols the connection arrangements within the digital switch 112 inaccordance with the call status. Moreover, the MSC control processor 110allocates and releases transcoder and combiner equipment 100 during callsetup and tear down from a corresponding resource pool (only one pieceof this transcoder and combiner equipment 100 is shown in the figure)

Transcoder and combiner equipment 100 is needed in order to convertbetween the typically μ-law encoded voice as used in the PSTN and thelow rate digital voice coding such as CELP used on the radio links. Inaddition to the transcoding function, transcoder and combiner equipment100 also implements the signal diversity combing in the uplink directionand signal duplication in the downlink direction. Moreover, transcoderand combiner equipment 100 synchronizes during soft handoff with signaldiversity combining the information flows to and from the participatingBSs, transmitted on the digital links 124,126 and switched throughdigital switch 112 via circuits 130,132 with the information flow to andfrom the PSTN, switched through the digital link 120, digital switch 112and circuit 134 (only 2-branch BS diversity is depicted in FIG. 2).

In the preferred embodiment of the present invention, the usercommunication signals, comprising digitized voice or data, multiplexedtogether with the signalling information related to this connection, arecarried in a digital, framed format suitable for the terrestrialtransmission links 124,126 between the BSs and the MSC. These frames aresubsequently referred to as transcoder and combiner frames. In additionto this user information, transcoder and combiner frames may alsocontain information supplied by the BSs which is relevant to the signalquality as used for the signal diversity combining within the MSC in theunlink direction. Moreover, the transcoder and combiner frames maycontain digital signals supplied by the BSs and the MSC which arerelevant for synchronizing the simultaneous links 124,126 between theBSs and the MSC during a soft handoff with signal diversity combining.

These transcoder and combiner frames arriving and leaving on thecircuits 130,132,134 are buffered in the digital memory 104 for theunlink and downlink direction, respectively. Digital processor 102 readsand writes cyclically the transcoder and combiner frames from and to thedigital memory 104. In the unlink direction, a signal quality indicationattached to the transcoder & combiner frames arriving from the circuits130,132 into the memory 104 is inspected and processor, 102 performs thediversity selection based on these indications. In the downlinkdirection, voice samples arriving from circuit 134 into the memory 104are transcoded and packed into transcoder & combiner frames by processor102.

Transcoder and combiner equipment 100 by means of the digital processor102 also respectively extracts, inserts, the user signallinginformation, from, into, the transcoder and combiner frames, and offers,receives, this signaling information to the MSC control processor 110via a circuit 140. By these means, the MSC control processor 110receives the MS signalling information such as pilot signal qualitymeasurement reports. Thus, the MSC control processor 110 possesses thenecessary information to initiate and terminate inter-MSC and intra-MSCsoft handoffs. Furthermore, by these means MSC control processor 110 canissue the appropriate handoff commands to the MS via circuits140,130,132 and links 124,126, as well as via the digital packet switch114 and link 122 to other MSCs, should this be required.

FIG. 3 shows an exemplary embodiment of a BS used in a CDMA cellulartelecommunications system to which the present invention relates.

Block 200 shows the apparatus required to support a single CDMAcommunication within a BS, referred to as CDMA channel equipment (onlyone is being shown).

In the uplink direction, the CDMA user communication signals arereceived from the digital CDMA radio link 230, demodulated by the CDMAdemodulator 202, de-interleaved and channel decoded by thede-interleaver and decoder 206, converted into transcoder and combinerframes and buffered for the terrestrial transmission within digitalmemory 210 and finally transmitted on the digital link 232 towards theMSC.

In the downlink direction, the transcoder and combiner frames arereceived from the MSC via digital link 232, buffered and converted intoa presentation appropriate for the BS within the digital memory 210,channel encoded and interleaved by the encoder & interleaver 208, CDMAmodulated by the CDMA modulator 204 and finally transmitted on thedigital radio link 230.

In the preferred embodiment of the present invention, the BS possesses anetwork hyphen independent timing source 220 which provides a referencesignal of high accuracy as required for efficient CDMA operation andutilized by the CDMA channel equipment 200. Such a timing source may bederived, e.g. from the GPS satellite signal a and can be providedglobally to each BS, thus enabling provision of a network of mutuallysynchronized BSs.

The BS further comprises a BS control processor 222. The BS controlprocessor 222 receives and transmits signalling information from and tothe digital link 232 connected to the MSC. The BS control processor 222performs the resource management of the BS, such as the allocation andreleasing CDMA channel equipment for user connections (calls). The BScontrol processor 222 thus responds to CDMA channel assignment requestsrelated to call setup as well as to CDMA channel assignment requestsrelated to soft handoff requests from the MSC.

The digital processor 212, in conjunction with the buffer memory 210,performs the packing and unpacking of the BS internal representation ofthe CDMA user communications signal and from transcoder & combinerframes respectively in the uplink, downlink directions. In the preferredembodiment of the present invention, the previously mentioned transcoderand combiner frames also contain information supplied by thede-interleaver and channel decoder 206 and provided to the digitalprocessor 212 which is indicative of the signal quality of the CDMAradio frames as received from the uplink CDMA radio link 230 and whichis used for the signal diversity combining within the MSC in the uplinkdirection.

As noted above, the present invention relates to soft handoff andmacrodiversity signal combining techniques. Within a cellulartelecommunications system of the present invention, soft handoff withsignal diversity combining can be performed even in the case that theinvolved BSs are connected to two different MSCs, in the followingreferred to as inter-MSC soft handoff with signal diversity combining.These MSCs are assumed to be connected either permanently or temporarilyvia digital links for the transmission of user communication signals andinter-MSC handoff a hyphen signalling information.

The cellular telecommunications system of the present invention furtherassumes the use of the mobile assisted soft handoff method as previouslydescribed for prior art systems, including the transmission of pilotsignals by all BSs as a downlink signal quality reference, appropriatepilot signal quality measurement and processing equipment within the MSand signalling means between the MS and the controlling MSC in order tocommunicate handoff trigger conditions, and handoff initiation,termination, commands based upon the pilot signal strength measurementsperformed by the MS. In the following the operation of the inventionwill be illustrated in the context of a CDMA system. However, theinvention is not intended to be restricted to CDMA systems and may beapplied in any multiple access system.

Initiation of inter-MSC soft handoff with signal diversity combining(FIG. 1)

In the following, it is assumed that an MS 30 communicates via a firstBS 24 connected to a first MSC 14, which provides access to the PSTN andother MSCs 10,12.

The inter-MSC soft handoff with signal diversity combining is initiatedwhen the MS 30 moves from the coverage area of the serving BS 24connected to the first MSC 14 to the coverage area of a second BS 22connected to a second MSC 12 and the MS pilot signal qualitymeasurements indicate that a soft handoff to the second BS 22 isappropriate. The MS signals this measurement indication via the first BS24 to the first MSC 14, including identification information of thesecond BS 22.

The first MSC 14 in turn detects from cellular configuration data thatBS 22 s connected to another MSC 12 and passes subsequently an inter-MSCsoft handoff request to this second MSC 12. This handoff requestidentifies the CDMA code channel and the frequency which the MS 30currently uses and additionally, the identity of the inter-MSC circuit50 reserved by MSC 14 for the this transaction.

MSC the 12 passes this handoff request further on to the BS 22 a afterreserving and switching through an appropriate circuit 72. BS 22analyzes the handoff request and if the requested resources areavailable allocates a further CDMA code channel to be used by MS 30 forthe downlink connection 82 which is signaled back to MSC 12 and MSC 14.BS 22 will also activate the downlink direction of connection 82 usingthe newly assigned CDMA code channel. BS 22 will further startdemodulating the CDMA uplink connection 82 using the CDMA contextinformation related to MS 30 and subsequently relaying the usercommunication signals via MSC 12 back to MSC 14 or diversity combining.BS 22 may signal the successful acquisition and reception of to CDMAunlink connection 82 to MSC 14 via MSC 12.

MSC 14 will send a handoff request via BS 24 to MS 30 including theidentity of the newly allocated CDMA code channel. MSC 14 will alsocommence diversity signal combining of the user communication signals onthe uplink once the user communications signals following the legs84--74, respectively 82--72--50, have been received in synchronism.

MS 30, after having received the handoff request a commences signaldiversity combining of the first CDMA downlink connection 84 and thesecond, newly allocated downlink connection 82. The successfulinitiation of the inter-MSC soft handoff with signal diversity combiningis then signaled from MS 30 to MSC 14.

Termination of inter-MSC soft handoff with signal diversity combining(FIG. 1)

The inter-MSC soft handoff with signal diversity combining is terminatedif the MS leaves completely the coverage area of one of theparticipating BSs and penetrates deeply into the coverage area of theother BS.

In the following, it is assumed that MS 30 has penetrated deeply intothe cell covered by BS 22 and therefore the pilot signal coming from BS24 has weakened below a predetermined threshold in the previouslydescribed inter-MSC sofhand off configuration. Thus, the leg 84--74shall be removed from the inter-MSC soft handoff with signal diversitycombining.

MS 30 will inform MSC 14 via a pilot signal quality measurement reportthat the signal coming from BS 24 has weakened below a predeterminedthreshold. MSC 14 decides to drop the leg 84--74, and thus to terminatethe inter-MSC soft handoff with signal diversity combining. To this end,MSC 14 sends a handoff termination signal via BS 24 and MSC 12--BS 22 toMS 30. MS 30 will stop the downlink demodulation diversity combining ofthe signal coming from BS 24 and will communicate from now on with BS 22only. MS 30 signals via BS 22 and MSC 12 the successful termination ofthe inter-MSC soft handoff with signal diversity combining back to MSC14 which in turn will request BS 24 to terminate the CDMA radio link 84and to release the corresponding resources. MSC 14 will also free theterrestrial link 74 and terminate the uplink diversity combining. Thiscompletes the termination of the inter-MSC soft handoff with signaldiversity combining. It should be understood that MSC 14 is still incharge of all control and signalling functions related to thecommunications with MS 30. Functionally, the leg 50--72--82 is treatedsimilarly to a direct MSC--BS interconnection (such as via BS 24); theonly difference is the additional relay functions performed by MSC 12.MSC 12 will therefore be completely transparent to all control andsignalling functions invoked by MSC 14, BS 22 and MS 30.

As a further example of the above a hyphen described inter-MSC handoffwith signal diversity combining, assume that the MS 30 has been movingback into the coverage area of the cell covered by BS 24 and thereforethe pilot signal quality coming from BS 22 has weakened below apredetermined threshold. Thus, the leg 82--72--50 shall be removed (notethat in this case the previously reserved circuit 50 is also removed)and leg 74-84 established using the above described soft handoff methodas follows:

MS 30 will inform MSC 14 via a pilot signal quality measurement reportthat the signal coming from BS 22 has weakened below a predeterminedthreshold. Subsequently, MSC 14 decides to drop the leg 82--72--50. Tothis end, MSC 14 sends a handoff signal via BS 24 and MSC 12--BS 22 toMS 30. MS 30 will stop the downlink demodulation diversity combining ofthe signal coming from BS 22 and will communicate from now on with BS 24only. MS 30 signals the successful inter-MSC soft handoff with signaldiversity combining back to MSC 14 which in turn will signal to MSC 12about the inter-MSC soft handoff. MSC 12, in turn, will request BS 22 toterminate the CDMA radio link 82 and to release the correspondingresources. MSC 14 will also free the terrestrial link 50 and terminatethe uplink diversity combining.

It should be understood that the above, described inter-MSC soft handoffwith signal diversity combining may be applied several times during anongoing communication. It should also be understood that more than twoBSs can participate during an inter-MSC soft handoff with signaldiversity combining. E.g., it may have been the case that in addition toBS 24 and BS 22 also BS 20 would have participated via link 70 and anadditional circuit on the link 50 in the inter-MSC soft handoff withsignal diversity combining. Also, more than two MSCs may participate inan inter-MSc soft handoff with signal diversity combining. E.g., it mayhave been the case that in addition to BS 24 and BS 22 another BS (notshown) connected to MSC 10 would have participated via link 48 in theinter-MSC soft handoff with signal diversity combining. Common to thesescenarios, is that MSC 14 is always in charge of all control andsignaling functions related to the communications with MS 30 and thusserves an anchor regarding all CDMA radio resource related functions.

It should also be understood that the method of the present inventioncan be readily applied to a TDMA cellular telecommunications system. Ina TDMA cellular telecommunications system, radio links 80, 82, 84, 86 of1, would be embodied as TDMA radio links in which several timeslots areused to provide communication channels to the system users. During softhandoff, in particular, during inter-exchange soft handoff withdiversity combining, two (or more) timeslots can be used to provide theconcurrent radio channels used by the MS and BSs involved in thehandoff. All the other mentioned characteristics o: the presentinvention remain the same for use of the invention a TDMA cellulartelecommunications system.

The previous description of the preferred embodiments are provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without the use of theinventive faculty. Thus, the present invention is not intended to belimited to the embodiment herein, but is to be accorded the widest scopeconsistent with the principles as novel features disclosed herein.

What is claimed is:
 1. In a cellular telecommunications system in whicha user mobile station relays user information radio signals via at leastone of a plurality of base stations and in which said base stationsfurther relay said user information signals via at least one of aplurality of mobile exchanges to and from another system user, the amethod for inter-exchange soft handoff with diversity combining,comprising the steps of:maintaining the relay of user informationsignals between a mobile station and a first mobile exchange via a firstbase station connected by a first connection to said first mobileexchange, said first mobile exchange further relaying the userinformation signals to and from another system user, said first mobileexchange controlling the user communications, establishing a furthersecond connection for further second relay of said user informationsignals between said mobile station and said first mobile exchange via afurther second base station connected to a further second mobileexchange, said second mobile exchange further relaying said usercommunications signals to said first mobile exchange, performingdiversity combining at said first mobile exchange of said userinformation signals as relayed from said mobile station to said firstmobile exchange via said first connection and said further secondconnection, said first mobile exchange offering the combined userinformation signal to said other system user, performing furtherdiversity combining at said mobile station of said user informationsignals as relayed from said other system user to said first mobileexchange and relayed further via said first connection and said furthersecond connection to said mobile station.
 2. The method according toclaim 1, in which the step of establishing said further secondconnection comprises the steps of:measuring the signal quality ofsurrounding base stations at said mobile station located in the coveragearea of said first base station and of said further second base station,signalling the measurement results of said measurements from said mobilestation to said first mobile exchange via said first base station usinga first radio connection, initiating, on the basis of said measurementresults an inter-exchange soft handoff with diversity combining towardssaid further second base station, said handoff initiation comprising thefurther steps of: determining that said second base station is connectedto said further second mobile exchange, transmitting an inter-exchangesoft handoff request signal from said first mobile exchange to saidsecond mobile exchange, said inter-exchange soft handoff request signalidentifying said radio connection used to relay said user informationradio signals between said mobile station and said first base station,allocating a connection between said first mobile exchange and saidsecond mobile exchange for the inter-exchange soft handoff withdiversity combining, allocating a connection between said second mobileexchange and said second base station for the inter-exchange softhandoff with diversity combining, allocating a further radio connectionbetween said mobile station and said second base station for theinter-exchange soft handoff with diversity combining, coupling the saidconnection between said first mobile exchange and said second mobileexchange with said connection between said second mobile exchange andsaid second base station and further coupling with said second radioconnection between said mobile station and said second base station, inorder to form a further second connection for further second relay ofsaid user information signals between said mobile station and said firstmobile exchange, transmitting an inter-exchange soft handoff responsesignal to said first mobile exchange, said inter-exchange soft handoffresponse signal, being responsive to said inter-exchange soft handoffrequest signal and said inter-exchange soft handoff response signalidentifying the said further second radio connection to be used to relaysaid user information radio signals between said mobile station and saidsecond base station, transmitting a soft handoff request signal betweensaid first mobile exchange to said mobile station via at least saidfirst base station, said soft handoff request signal identifying saidfurther second radio connection used to relay said user informationradio signals between said mobile station and said second base station,relaying said user information signals between said mobile station andsaid first mobile exchange concurrently via said first connectionincluding said first base station and via said second connectionincluding said second base station and said second mobile exchange. 3.The method according to claim 1, further comprising the step ofinter-exchange soft handoff termination, including the stepsof:measuring the signal quality of surrounding base stations at saidmobile station located in the coverage area of said first base stationand of said further second base station, signalling the measurementresults of said measurements from said mobile station to said firstmobile exchange via at least one of said first connection or saidfurther second connection, terminating, on the basis of said measurementresults said inter-exchange soft handoff condition, said handofftermination comprising the further steps of: transmitting a soft handofftermination signal from said first mobile exchange to said mobilestation via at least one of said first base station or said second basestation, transmitting a soft handoff termination request signal fromsaid first mobile exchange to said second mobile exchange, terminatingthe said diversity combining at said first mobile exchange, terminatingthe said diversity combining at said mobile station, terminating theradio connection between mobile station and said first base station,terminating the connection between said first base station and saidfirst mobile exchange, maintaining the connection between said mobilestation and said first mobile exchange via said second base station andsaid second mobile exchange, said first mobile exchange stillcontrolling the user communication.
 4. The method according to claim 1or 2 wherein said user information radio signals relayed between saidmobile station and said first and said second base station are CDMAspread spectrum modulated.
 5. The method according to claim 1 or 3wherein the user information radio signals relayed between said mobilestation and said first and said second base station are CDMA spreadspectrum modulated.
 6. The method according to claim 1 or 2 wherein saiduser information radio signals relayed between said mobile station andsaid first and second base station are TDMA modulated.
 7. The methodaccording to claim 1 or 3 wherein the user information radio signalsrelayed between said mobile station and said first and second basestation are TDMA modulated.
 8. In a cellular telecommunications systemcomprising a plurality of mobile exchanges, a plurality of base stationsand a plurality of mobile stations roaming throughout the system area,in which a user mobile station relays user information radio signals viaat least one of a plurality of base stations and in which said basestations further relay said user information signals via at least one ofa plurality of mobile exchanges to and from another system user, asystem of inter-exchange soft handoff with diversity combining, saidsystem comprising:means at each mobile station of said plurality ofmobile stations and at each base station of said plurality of basestations and at each mobile exchange of said plurality of mobileexchanges, to maintain the relay of user information signals between amobile station and a first mobile exchange via a first base stationconnected by a first connection to said first mobile exchange, furthermeans at said first mobile exchange for further relaying the userinformation signals to and from another system user, further means atsaid first mobile exchange for controlling the user communications,means at each mobile station of said plurality of mobile stations and ateach base station of said plurality of base stations and at each mobileexchange of said plurality of mobile exchanges for establishing afurther second connection for further second relay of said userinformation signals between said mobile station and said first mobileexchange via a further second base station connected to a further secondmobile exchange, said second mobile exchange comprising further meansfor further relaying said user communications signals to said firstmobile exchange, means at said first mobile exchange for performingcombining of said user information signals as relayed from said mobilestation to said first mobile exchange via said first connection and saidfurther second connection, said first mobile exchange comprising furthermeans for offering the combined user information signal to said othersystem user, means at said mobile station for Performing furtherdiversity combining of said user information signals as relayed fromsaid other system user to said first mobile exchange and relayed furthervia said first connection and said further second connection to saidmobile station.
 9. The system according to claim 8, furthercomprisingmeans at each base station of said plurality of base stationsfor transmitting a pilot signal indicative of each base station oforigin, means at said mobile station for measuring the signal quality ofsaid pilot signals of surrounding base stations, said mobile stationslocated in the coverage area of said first base station and of saidfurther second base station, means at said mobile station and at saidfirst base station and at said first mobile exchange for signalling themeasurement results of said pilot signal measurements from said mobilestation to said first mobile exchange via said first base station usinga first radio connection, means at said mobile station and at said firstand said second base station and at said first and said second mobileexchange for initiating, on the basis of said measurement results aninter-exchange soft handoff with diversity combining towards saidfurther second base station, said handoff initiation necessitating thefurther means of: means at said first mobile exchange for determiningthat said second base station is connected to said further second mobileexchange means at said first, respective second mobile exchange fortransmitting respective receiving an inter-exchange soft handoff requestsignal from said first mobile exchange to said second mobile-exchange,said inter-exchange soft handoff request signal identifying said radioconnection to be used to relay said user information radio signalsbetween said mobile station and said first base station, means at saidfirst and said second mobile exchange for allocating a connectionbetween said fist mobile exchange and said second mobile exchange forthe inter-exchange soft handoff with diversity combining, means at saidsecond base station and at said second mobile exchange for allocating aconnection between said second mobile exchange and said second basestation for the inter-exchange soft handoff with diversity combining,means at said mobile station and at said second base station forallocating a further radio connection between said mobile station andsaid second base station for the inter-exchange soft handoff withdiversity combining, means at said second base station and at saidsecond mobile exchange for coupling the said connection between saidfirst mobile exchange and said second mobile exchange with saidconnection between said second mobile exchange and said second basestation and further coupling with said second radio connection betweensaid mobile station and said second base station, in order to form afurther second connection for further second relay of said userinformation signals between said mobile station and said first mobileexchange, means at said second mobile exchange for transmitting andmeans at said first mobile exchange for receiving an inter-exchange softhandoff response signal, said inter-exchange soft handoff responsesignal, being responsive to said inter-exchange soft handoff requestsignal and said inter-exchange soft handoff response signal identifyingthe said further second radio connection used to relay said userinformation radio signals between said mobile station and said secondbase station, means at said first mobile exchange for transmitting asoft handoff request signal between said first mobile exchange to saidmobile station via at least said first base station, said soft handoffrequest signal identifying said further second radio connection to beused to relay said user information radio signals between said mobilestation and said second base station, further means at said mobilestation and said at least first base station to receive said handoffrequest signal and to establish said second radio connection, means atsaid mobile station and at said first and said second base station andat said first and said second mobile exchange for relaying said userinformation signals between said mobile station and said first mobileexchange concurrently via said first connection including said firstbase station and via said second connection including said second basestation and said second mobile exchange.
 10. The system according toclaim 8 or 9, wherein the cellular system a CDMA system or a TDMAsystem.